We will measure the functional caliber of alveolar septal microvessels in isolated rat lungs which have been prepared so that lung inflation pressure exceeds perfusion pressure (zone I ). This causes the microvessels to compress nearly to the point of collapse. The measurements will be made by perfusion isolated rat lungs with fluorescent latex particles of specific diameters (0.010- 10.0 micrometers). The lungs will then be frozen, fixed by freeze substitution and embedded. Sections will be examined using a laser confocal fluorescence microscope. From the images, latex particle density will be measured in individual alveolar septa. Data will be compared from lungs perfused with different particle diameters and used to calculate the functional septal microvascular syncytial caliber under the conditions in which the lungs were perfused. Using a newly developed method, we plan to reconstruct three dimensionally the perfusion characteristics of intravenous ruscitation and hemoglobin solutions in isolated rat lungs. It is of medical importance to follow these pathways to see if perfusion exists at the capillary level. If these solutions can enter capillaries under high lung inflation pressures, it will be very useful as an oxygen carrying agent in treating hypovolemic shock patients.